Light-Emitting Diode (LED) non-impact printing apparatus uses an LED printhead as a light source within the imaging device. The printhead is essentially a line of LEDs the width of a page and is used for printing on ordinary paper using electrophotographic reproduction apparatus. The LED printhead is solid state and has no moving parts. The LED printhead creates the image on the print drum as the drum moves down.
Light-Emitting Diode (LED) printheads (also referred to as writers) typically contain a series of contiguous LED linear array chips which are imaged onto a photoconductor receptor by means of a gradient-index lens (e.g., Selfoc “self focusing” lens available from Nippon Sheet Glass Co.). Gradient-index (GRIN) lenses have a graded refractive index that is a maximum at the center of the lens and decreases quadratically towards the outer edge. This smoothly varying index of refraction causes incident rays of light to refract within the lens and converge towards a point of focus. The LEDs associated with each LED array chip are typically activated by a driver integrated circuit that provides a prescribed amount of current to a given LED for a prescribed duration.
It is known in the art of non-impact printing that correction of the LED recording elements is often required due to non-uniformity in light output of these elements. Typically, a non-uniformity correction look-up table (LUT) is provided to adjust exposure times so that at any required gray level all LEDs can be enabled to output a uniform amount of exposure energy. This can be achieved by adjusting exposure times and/or intensities so that weaker emitters are enabled for longer exposure times than stronger emitters in order for the exposure energy from each emitter to be uniform. The LUT is typically a 256 byte array for each color band containing the output values that are computed. Input pixel values operate as an index into the table. LUT functionality provides a procedure that computes the output values once. Thereafter, every pixel call returns the corresponding computed value. For a typical LED printhead of approximately 4000 LEDs, a LUT memory can represent the grading of the LEDs into 256 categories according to their respective brightness levels.
LED printheads typically have pixel irradiance non-uniformity on the image plane due to pixel-to-pixel brightness differences for LED emitters as well as non-uniformity of transmission. Even if the same current is driven through each LED, LEDs respond differently and their light outputs vary. Exposure uniformity correction either via exposure time modulation, such as using pulse-width-modulation (PWM), or pixel drive-current trimming, has been used before to correct for the pixel intensity non-uniformity on the image plane. For the typical PWM or drive-current trimming uniformity correction scheme, the assumption is that only the integrated irradiance of the pixel on the image plane is assumed to be non-uniform from pixel to pixel. These uniformity-correction schemes do not account for variations in the pixel size, spacing, or the LED on-time characteristics.
In the case where the size of an LED pixel irradiance spot on the image plane is different than the nominal spot size (either due to modulation transfer function (MTF) differences across the width of the optics, or due to the LED emitter mask size/dopant variation between LED arrays), then the typical non-uniformity correction method that uses integrated pixel-intensity correction only will have some residual exposure non-uniformity due to the pixel-size variation effect which creates variation in exposure density (area coverage).
Although LEDs are uniformly spaced or pitched because of the accuracy of the photolithographic fabrication process, the spacing between LEDs at the end of butting LED arrays does not have this uniformity of pitch. Therefore, the effective exposure power density can also vary across the LED arrays due to pixel-spacing variations caused by LED array butting-gap deviations, or optics-induced image-shift errors. The pixel-spacing variation creates a difference between the actual pixel spacing and the nominal pixel pitch within an array. In the case of a pixel-spacing variation due to a spacing error at the butting gap between end pixels of adjacent LED arrays, the exposure-power density across the gap region will be different than the power density at nominal pitch. Under this situation, an exposure-density modification for correcting for the butting-gap pitch error can be made by altering the pixel-brightness of the end pixels located at the array gap. This essentially maintains an effective equivalent exposure energy density at the butting gap compared to the rest of the pixels within an array. An analogous case can be made for other pixel-spacing errors caused by the Selfoc lens. The misalignment of the gradient-index rods in the Selfoc lens can create an image-shifting error which can lead to an altering of the effective pixel spacing, thus creating a variation in the exposure density.
Pixel-exposure variation can also be attributed to variations in the on-time characteristics due to dynamic effects where the LED rise time and steady-state level are affected by the number of pixels activated. Specifically, the LED and driver combination can be subject to current-starvation characteristics whereby the resultant integrated exposure of an individual LED pixel can change (typically decreases) depending on the total number of LEDs that are activated. When a single LED is activated, the overall printhead has minimal current loading and the LED is not “starved” for current. When a majority of LEDs are activated, as is the case in flat-field printing, the current loading is high and the individual LED may degrade slightly in light output due to current starvation. The level of degradation may or may not be uniform. Even with constant-current driver designs or current-compensating driver designs, there can still exist noticeable levels of nonuniformity due to individual component variation and printhead assembly manufacturing variation.
The dynamic effects of current starvation can affect the exposure by reducing not only the overall steady-state power output, but also by altering the LED rise time to create a nonlinearity between the integrated LED on-time and the integrated pixel exposure. In conventional on-time based uniformity-correction schemes, brighter LEDs are programmed to have shorter on-times than dimmer LEDs. Therefore, in a typical flat-field printing situation, the brighter LEDs are affected by the current-starvation effects more than the dimmer LEDs which are compensated with a longer activation time. The uniformity can be degraded in the case of high brightness pixels being activated for very short on times for low gray level printing.
To resolve these problems in the art, a more effective uniformity-correction method must take into consideration not only the LED pixel irradiance differences, but also the pixel-size variation, the pixel-spacing variation, and the dynamic current-starvation effects to correct for the residual exposure non-uniformity on the image plane.